netdev - Re: [Patch net] lwt: disable BH too in run_lwt

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Message-ID: <20201202171032.029b1cd8@kicinski-fedora-pc1c0hjn.DHCP.thefacebook.com>
Date:   Wed, 2 Dec 2020 17:10:32 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev@...r.kernel.org, Dongdong Wang <wangdongdong@...edance.com>,
        Thomas Graf <tgraf@...g.ch>, bpf@...r.kernel.org,
        Cong Wang <cong.wang@...edance.com>
Subject: Re: [Patch net] lwt: disable BH too in run_lwt_bpf()

On Tue,  1 Dec 2020 11:44:38 -0800 Cong Wang wrote:
> From: Dongdong Wang <wangdongdong@...edance.com>
> 
> The per-cpu bpf_redirect_info is shared among all skb_do_redirect()
> and BPF redirect helpers. Callers on RX path are all in BH context,
> disabling preemption is not sufficient to prevent BH interruption.
> 
> In production, we observed strange packet drops because of the race
> condition between LWT xmit and TC ingress, and we verified this issue
> is fixed after we disable BH.
> 
> Although this bug was technically introduced from the beginning, that
> is commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure"),
> at that time call_rcu() had to be call_rcu_bh() to match the RCU context.
> So this patch may not work well before RCU flavor consolidation has been
> completed around v5.0.
> 
> Update the comments above the code too, as call_rcu() is now BH friendly.
> 
> Cc: Thomas Graf <tgraf@...g.ch>
> Cc: bpf@...r.kernel.org
> Reviewed-by: Cong Wang <cong.wang@...edance.com>
> Signed-off-by: Dongdong Wang <wangdongdong@...edance.com>
> ---
>  net/core/lwt_bpf.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
> index 7d3438215f32..4f3cb7c15ddf 100644
> --- a/net/core/lwt_bpf.c
> +++ b/net/core/lwt_bpf.c
> @@ -39,12 +39,11 @@ static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
>  {
>  	int ret;
>  
> -	/* Preempt disable is needed to protect per-cpu redirect_info between
> -	 * BPF prog and skb_do_redirect(). The call_rcu in bpf_prog_put() and
> -	 * access to maps strictly require a rcu_read_lock() for protection,
> -	 * mixing with BH RCU lock doesn't work.
> +	/* Preempt disable and BH disable are needed to protect per-cpu
> +	 * redirect_info between BPF prog and skb_do_redirect().
>  	 */
>  	preempt_disable();
> +	local_bh_disable();

Why not remove the preempt_disable()? Disabling BH must also disable
preemption AFAIK.

>  	bpf_compute_data_pointers(skb);
>  	ret = bpf_prog_run_save_cb(lwt->prog, skb);
>  
> @@ -78,6 +77,7 @@ static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
>  		break;
>  	}
>  
> +	local_bh_enable();
>  	preempt_enable();
>  
>  	return ret;